ABSTRACT: In the context of increased application of proton exchange membrane electrolyzers, the development of cheap and long-lived transition-metal hydrogen evolution reaction electrocatalysts is required to circumvent nonsustainable platinum-based electrocatalysts. Herein we report the synthesis and characterization of robust iron? sul?de nanoparticles from low-cost precursors (i.e., FeCl3 and thiourea) using an easily scalable soft synthesis technique, which can achieve electrocatalysis. In the series of nanoparticles studied, we show that pyrite FeS2 is the most active, in comparison with greigite Fe3S4 and pyrrhotite Fe9S10, in a three-electrode electrochemical cell, the electrocatalysis starting at an overpotential of ?180 mV. These three materials exhibit a very stable behavior during the catalysis, with no activity decrease for at least 5 days. FexSy catalysts have been tested in a PEM electrolysis single cell, and pyrite FeS2 allows achievement of a current density of 2 A/cm2 at a voltage of 2.3 V. KEYWORDS: hydrogen evolution reaction, iron sul?de nanoparticles, electrocatalysis, nanoparticle Mos ? sbauer spectrometry, electrode modi?cation

1. INTRODUCTION The supply of su?cient energy without creation of additional CO2 in the atmosphere is one of the most important issues facing humanity at this time. On the basis of the current consumption rate, the available reserves of fossil fuels will be unable to keep up with the increase in demand that is expected, while the global population is estimated to grow to 9.4 billion by 2050.1 Clean, sustainable, and economical hydrogen production is currently a crucial challenge, as hydrogen may become a major energy carrier in the near future and reduce our fossil fuel dependence.2 Proton exchange membrane (PEM) technologies are of great interest, since they are well suited for applications in mobile and portable devices.3 Regarding PEM water electrolyzers, they are called to play a key role in the future in the management of renewable energy sources, since they are capable of operating at intermittent and ?uctuating current/potential and store renewable energy as hydrogen; nevertheless, the need for noble-metal catalysts both at the anode and the cathode limits the development and commercialization of both fuel cell and electrolyzer technologies.? XXXX American Chemical Society

Among the di?culties encountered in switching to a hydrogen economy, the problem of the large-scale electrochemical production of molecular hydrogen is still facing some hardship, for the most part due to the requirement of platinumgroup metals (PGM), which are capable of driving the hydrogen evolution reaction (HER) close to its thermodynamic potential. Indeed, platinum is the reference catalyst both in PEM fuel cells (FC) and in PEM electrolyzers.4?6 To replace noble metals for the HER from neutral or acidic aqueous solution, heterogeneous non-precious-metal electrocatalysts showing high activity toward the HER based on nanocrystals or electropolymerized ?lms have been reported in the past few years.7 These electrocatalysts have been prepared using cheap and abundant transition-metal materials, such as molybdenum sul?des,8 tungsten sul?de,9 cobalt sul?de,10 nickel?molybdenum alloys,11 cobalt phosphates,12 iron phosphide,13 nickel phosphide,14 iron sul?des,15 or ?rst-row transition-metalReceived: October 30, 2015 Revised: February 26, 2016

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ACS Catalysis dichalcogenides.16 Nevertheless, the number of noble-metalfree catalysts investigated in real PEM electrolyzers for HER is scarce.5,17 As an example, Millet et al. reported in situ characterization of tungstosilicic acid (α-H4SiW12O40) and Co complex/Vulcan XC-72 cathode catalysts and Ir0 anode catalysts and measured potential values of 1850 and 2100 mV at 1 A/cm2, respectively.18 We recently reported that robust iron sul? de Fe 9S 10 pyrrhotite-type nanoparticles were able to electrocatalyze the HER in neutral water at a mild overpotential with a catalytic activity exceeding 5 days without any sign of decomposition.19 A striking advantage of this type of electrocatalyst is the ubiquity of iron sul?de minerals in nature. Pyrite FeS2 is the most abundant mineral on the Earth’s surface, and pyrrhotite Fe9S10 is the most abundant iron sul?de in the Earth and solar system.20 Herein we describe the synthesis and characterization of di?erent stoichiometries of iron sul?de FexSy nanomaterials and their activity toward the HER. Pyrite FeS2, greigite Fe3S4, and pyrrhotite Fe9S10 crystalline phases were prepared using a soft synthetic method: the polyol route. Morphological and electronic properties of the prepared nanoparticles were characterized, as well as their electrochemical properties. Finally their performances were investigated in situ in a PEM electrolyzer single cell.

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samples are presented in Figure 2. The greigite sample is formed of micrometer-sized gypsum ?owerlike particles

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of Iron Sul?de Nanoparticles. Transition-metal chalcogenide nanoparticles have attracted much attention over the past few years, due to their applications in the ?elds of energy conversion and storage,21 resulting in the proliferation of new synthetic methods to prepare metal sul?de nanoparticles. We decided to use the polyol method for the preparation of FexSy materials. This method allows the preparation of a large variety of materials (metals, oxides, sul?des, or alkoxides) with submicrometer to nanometer particle size at relatively low temperatures and pressure conditions (i.e., up to 250 °C under atmospheric pressure) in diol solvents.22 Since the iron? sulfur binary diagram is complex,23 various procedures have been used to obtain pure phases of di?erent FexSy nanoparticle stoichiometries to investigate the in?uence of (i) iron oxidation state, (ii) sulfur oxidation state, and (iii) crystal structure on the catalysis of the HER (Figure 1). Pyrite FeS2 and greigite Fe3S4

Figure 1. Iron and sulfur formal oxidation states.

particles were prepared using the same precursors, thiourea and anhydrous iron(III) chloride, and identical experimental conditions. The Fe/S ratio was simply tuned by changing the nature of the polyol solvent: 1,2-propanediol was used in the case of the pyrite FeS2 and ethylene glycol for the synthesis of the greigite Fe3S4 (see the Supporting Information for synthetic and characterization details). The preparation of pyrrhotite Fe9S10 was reported in a previous publication.19 The morphologies of the as-prepared nanopowders were analyzed both by SEM and TEM. Typical SEM images of FexSy2627

consisting of thin platelets with a very high aspect ratio (thickness from 10 to 30 nm, length a few hundreds of nanometers). Closer inspection of the edge of the particles on TEM images revealed that they comprise smaller particles (a few tens of nanometers) with a platelet or a needle shape. Similarly pyrite particles have a hierarchical morphology consisting of large micrometer-sized spheres of aggregated smaller particles (ca. nanometer in size). These morphologies are typical of iron sul?de nanoparticles obtained by the polyol process.24?26 The crystal phase of the as-prepared powders was determined by X-ray di?raction (XRD). The di?raction patterns of the two samples are shown in Figure S5 in the Supporting Information. The di?raction pattern for the sample prepared from ethylene glycol can be indexed as cubic greigite Fe3S4 (Fd3?m space group) and those prepared from 1,2propanediol as cubic pyrite FeS2 (Pa3? space group) without detectable impurities of other Fe?S or Fe?O compounds. For the pyrite sample, re?nement of the lattice parameter led to a = 5.439 ?, in good agreement with the value given in the standard card JCPDS 98-004-1995 (a = 5.441 ?). The mean crystallite size was estimated to be 55 nm by Rietveld re?nement with the help of Maud software.19 For the greigite sample, the re?ned value of the lattice parameter was found to be a = 9.865 ?, consistent with the 9.867 ? value given in the standard card JCPDS 98-004-2535. The mean crystallite size was calculated to be around 46 nm. To gain a better understanding of the electronic structure of the materials, the oxidation and magnetic states of the iron atomes were probed using 57Fe Mo?ssbauer spectrometry at 300 and 77 K. As illustrated in Figure 3a and Figure S3 in the Supporting Information, spectra of pyrite and greigite recorded at 300 K look very di?erent. The former exhibits a typical quadrupole split symmetric doublet with an isomer shift of 0.32 mm/s (300 K) and 0.41 mm/s (77 K) and a quadrupole splitting of 0.59 mm/s (300 K) and of 0.635 mm/s (77 K) (Figure S4 in the Supporting Information). These data are in fair agreement with the known description of low-spin Fe2+ ions in pyrite.27 For the greigite sample, at 300 and 77 K (Figure 3a,b), the spectra clearly exhibit a magnetic hyper?ne structure with broadened and asymmetrical lines in addition toDOI: 10.1021/acscatal.5b02443 ACS Catal. 2016, 6, 2626?2631

a central quadrupolar doublet. Its modeling scheme consists of two magnetic sextets and a central doublet typical of a paramagnetic iron state (see Table 1). The doublet has the smallest contribution to the whole signal. The magnetic sextet can be decomposed into two components, while the corresponding hyper?ne values are given in Table 1. The hyper?ne ?eld values are similar to those described by Lyubutin et al. for synthetic greigite nanoparticles of 18 nm size24 and by Li et al. for high-quality greigite.28 In contrast, the isomer shift values signi?cantly di?er from those encountered by these authors but remain typical of tetrahedral and octahedral Fe species, respectively. This prevents the conclusion of a localized mixed valence state structure. In addition, the proportions of these two components are quite consistent with those of greigite. We also recorded a Mo?ssbauer spectrum at 77 K in the presence of an external magnetic ?eld of 8 T applied parallel to the γ beam using a cryomagnetic device. As illustrated in Figure 3c, outer lines belonging to two well-de?ned magnetic sextets which exhibit low-intensity intermediate lines are clearly distinguished. This hyper?ne structure is due to the weakly canted ferrimagnetic structure of the greigite sample. In addition, this in-?eld spectrum can be rather well described by means of three components: the re?ned values, which are given in Table 1, con?rm unambiguously the presence of Fe3+

species in tetrahedral sites and close intermediate Fe2.5+ species in octahedral sites. It is important to emphasize that our results obtained in the presence of an external ?eld become perfectly consistent with those obtained by Li et al.28 One should conclude that electronic relaxation phenomena exist which are canceled in the presence of a large magnetic ?eld in the greigite sample: such a feature which has never been observed in greigite samples originates probably from the defective atomic nature of the nearby sul?de. Indeed, the occurrence of vacancies through the crystalline structure justi?es the imperfect description of the experimental X-ray pattern and may in?uence the electronic properties, although a nonstoichiometric structure would a?ect the values of the isomer shift. 2.2. Electrochemical Characterization by Rotating Disk Electrode. FexSy nanoparticles were mixed with 20% of carbon black and coated with Na?on polymer electrolyte on a glassy-carbon electrode to a?ord uniform ?lms with a mean thickness of 1?2 μm. Electrochemical measurements were recorded at pH 7.0 in 0.1 mol/L phosphate bu?er and showed the typical current increase for a catalytic process at the surface of the electrode for the three FexSy species (Figure 4), in line

with the ?rst analysis reported.19 For pyrite FeS2 the catalytic production of molecular hydrogen from an overpotential of 220 mV was con?rmed by gas chromatography measurements. Thus, the onset overpotential is improved by ca. 130 mV in comparison to the pyrrhotite Fe9S10 nanoparticles reported

a Isomer shift value quoted with respect to that of α-Fe at 300 K. bQuadrupolar splitting or quadrupolar shift. cE?ective ?eld obtained from in-?eld Mo?ssbauer spectrum. dθ corresponds to the angle de?ned by the direction of the e?ective ?eld and the external magnetic ?eld. eHyper?ne ?eld obtained from zero-?eld Mo?ssbauer spectrum or estimated from the e?ective ?eld (see footnote c) using the following relation: Bhf2 = Beff2 + Bapp2 ? 2BeffBapp cos θ. fProportions of Fe species taken as the relative absorption areas, assuming the same f recoilless factor for the di?erent Fe species.

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ACS Catalysis previously, with the pyrite catalyst being slightly more active than greigite. To evaluate the stability of the coated ?lms, the electrodes were poised galvanostatically at ?10 mA/cm2, the approximate current density expected for an integrated solar water splitting device under 1 sun illumination operating at 10% solar to fuel e?ciency,29 over 2 h and showed no overpotential rises due to decomposition processes (Figure S7 in the Supporting Information). Controlled-potential electrolysis at an overpotential of η = 350 mV was performed for the three FexSy materials to evaluate their robustness and e?ciency. The Faradaic yield was found to be almost quantitative (≥0.95) for molecular hydrogen evolution (see the Supporting Information for details). As reported earlier,19 the pyrrhotite sample gave 4.2 mmol of H2 (250 C) after 5 days of electrolysis, and the greigite sample gave 1.3 mmol of H2 (250 C) after 5 days of electrolysis; for the pyrite sample, the catalysis was so e?cient that after 28 h 5.2 mmol of H2 (1100 C) was produced and the solution was no longer bu?ered (?nal pH 11.7). Postelectrolysis XRD analysis was performed on the three samples, and as noticed previously for pyrrhotite Fe9S10 nanoparticles,19 no structural change was observed for greigite Fe3S4 (see the Supporting Information for experimental details and ?gures). Fe9S10 and Fe3S4 were also monitored by microscopy, which showed no alteration of the morphology. For pyrite FeS2, an amorphization process was detected by XRD after several hours of electrolysis, but with no loss of activity of the catalysis (Figure S5 in the Supporting Information). This point is currently being investigated by the means of X-ray absorption spectroscopy. 2.3. In Situ Electrochemical Characterization. 2.3.1. InElectrolyzer Testing. Membrane?electrode assemblies (MEA) were prepared using pyrite, pyrrhotite, or greigite as the cathode catalyst and tested in an electrolysis single cell. The catalysts were not supported but were mixed with 20% of carbon black. For comparison, iV curves were also recorded on MEAs prepared with 0, 1, 2, and 4 mg/cm2 of carbon black at the cathode without iron sul?de nanoparticles (Figure S12 in the Supporting Information). Na?on 115 (125 μm) was used as the membrane and IrO2 as the anode catalyst. Typical iV curves obtained with pyrite, pyrrhotite, and greigite at 80 °C and atmospheric pressure with 2 mg/cm2 of carbon black are displayed in Figure 5.

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For the same catalyst loading of ?4 mg/cm2, as observed during ex situ electrochemical measurements, the performance of pyrite is greater than that of greigite and pyrrhotite. In the activation zone, typically j < 200 mA/cm2, the rapid voltage increase is attributed to kinetic polarization. In the present study, since the anode catalyst remained unchanged, the di?erence observed at low current density can be attributed to changes in the cathode catalyst. Even if the Tafel slope is in principle attributed to both anodic and cathodic overpotential, when a new anode catalyst is studied, the Pt overpotential can be neglected. Nevertheless in our case, IrO2 overpotential has to be taken into account. The Tafel slope of each MEA, including the contribution of both electrodes, as well as measured potential values at 1 and 2 A/cm2 for the di?erent catalyst-based MEAs at 80 °C are summarized in Table 2. Table 2. Summary of Electrolyzer Characterization of Prepared MEAs with Na?on 115 Membrane and IrO2 as Anode Catalyst at 80 °Ccathode catalyst U @ 1 A/cm2 a U @ 2 A/cm2 a Tafel slopeb Pt/C 1643 1830 51 pyrite/C 2101 2233 204 greigite/C 2130 2287 224 pyrrhotite/C 2158 2326 234

a In mV. bTafel slopes (in mV/dec) of all prepared catalysts were obtained by linear regression from iV curve of electrolyzer cells.

The activation required for reduction of proton is much larger in the case of iron sul?de based catalysts than for platinum. The Tafel slope of the three FeS-based catalysts is also larger than that of platinum. For a given current density, the potential obtained with the pyrite cathode based MEA is lower than of greigite, itself being lower than that of pyrrhotite. Nevertheless, all three catalysts allow ?2100 mV at 1 A/cm2 to be reached. The temperature e?ect on the cell performance was evaluated for the pyrite-based MEA (Figure 6). As the

temperature was increased from 80 to 120 °C, signi?cant enhancement in the electrode kinetics could be observed. Above 100 °C membrane and/or ionomer dehydration occurred, resulting in a decrease in proton conductivity and greater iR loss in the linear region of the IV curve, and a greater potential value for a given current density was observed.DOI: 10.1021/acscatal.5b02443 ACS Catal. 2016, 6, 2626?2631

ACS Catalysis Both ex situ and in situ electrochemical experiments led to the conclusion that pyrite (FeS2) is more active than greigite (Fe3S4), itself being more active than pyrrhotite (Fe9S10). As reported for other sul?de materials (MoSx, CoSx), sul?de or disul?de moieties present in the three FexSy materials may play the role in the catalytic sites.8,30 Indeed it has been ascertained that a higher density of S atoms at the material surface, and in particular the presence of S22? ions, is responsible for high catalytic performance. Since the S/Fe ratio is greatly favored for pyrite (2) in comparison to greigite (1.33) and pyrrhotite (1.11), for the same catalyst loading the density of possible catalytic sites is higher in pyrite. In addition, as can be seen in Figure 1, the iron centers in pyrite are in the +II oxidation state and the sulfur ligands are bridging disul?des μ-η3:η3-S22?, while for the pyrrhotite phase the iron centers are in the +II oxidation state and the sulfur ligands are bridging sul?des μ-η2-S2? and for the greigite phase the iron centers are in a mixed-valence state with one FeII and two FeIII with bridging sul?des μ-η3-S2?. The observation made on these FexSy materials corroborate results presented in the literature on other MxSy species and seems to con?rm the important role of S22? species present initially in the material as well as the high density of S atoms. 2.3.2. Stability and Microstructure of the Catalyst Layer. MEAs were observed using SEM-FEG after in situ testing (Figure 7). The thickness of the IrO2 catalyst layer is ?6 μm,

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term electrolysis. In situ electrolyzer characterization has been carried out successfully and showed that these materials hold promise as cathode catalysts in such devices. Our future work will focus on a mechanistic understanding of proton reduction by iron sul?de nanoparticles and the improvement of the e?ciency of these electrocatalysts.

4. EXPERIMENTAL SECTION4.1. Synthesis. For greigite Fe3S4 nanoparticles, 0.702 g (9.20 mmol, 2.00 equiv) of thiourea was dissolved in 90 mL of ethylene glycol in a four-necked round-bottom ?ask equipped with a re?ux condenser, a mechanical stirrer, and a thermocouple. The solution was heated at 180 °C (temperature ramp of 6 °C/min) with stirring at 100 rpm. A solution of 0.746 g (4.60 mmol, 1.00 equiv) of anhydrous FeCl3 dissolved in 10 mL of ethylene glycol was then injected in one shot. The mixture was stirred at 100 rpm at 180 °C for 17 h. The precipitate was centrifuged (15 min at 8500 rpm) to remove the polyol solvent and washed with ethanol (?ve times; 15 min at 22000 rpm). The black powder corresponding to greigite was stored under vacuum to avoid surface oxidation. Yield: 40% (based on iron reactant). For pyrite FeS2 nanoparticles, a procedure analogous to that employed for the preparation of greigite was adopted but the the polyol solvent was changed to 1,2-propanediol. Yield: 30% (based on iron reactant). 4.2. Preparation of Catalyst Inks and Fabrication of Membrane?Electrode Assemblies (MEAs). The anode catalyst layer consisted of IrO2 catalyst (2.2 mg/cm2) and 20 wt % Na?on ionomer (5 wt % solution, Aldrich). The cathode catalyst layer consisted of pyrrhotite Fe9S10, greigite Fe3S4, or pyrite FeS2 catalyst (4 mg/cm2), 14 wt % carbon Vulcan XC72R (Cabot), and 30 wt % Na?on ionomer (5 wt % solution, Aldrich). Catalyst inks were prepared using a 3/1 isopropyl alcohol/water solution. Na?on 115 membrane was boiled for 1 h in 3 wt % H2O2, rinsed with deionized water, soaked for 1 h in 50 vol % HNO3, rinsed with deionized water, boiled for 1 h in 1 mol/L H2SO4, and rinsed with deionized water. The membrane was dried at 80 °C for 4 h. For anode preparation, 25 mg of IrO2 and 152 μL of Na?on (5 wt % Aldrich) were mixed with 1 mL of deionized water and 3 mL of isopropyl alcohol. The suspension was sonicated for 1 h. The ink was then sprayed using an Aerograph onto a 6.25 cm2 Te?on sheet until a 2 mg/cm2 catalyst loading was reached. The electrode was transferred onto a pretreated Na?on 115 membrane using the following procedure: the MEA was heated in a hot press at 80 °C, the pressure was then increased to 80 kg/cm2, the temperature was then increased to 135 °C, and on stabilization the pressure was increased to 160 kg/ cm2; the MEA was left at 135 °C and 160 kg/cm2 for 90 s. For cathode preparation, 50 mg of catalyst, 12.5 mg of carbon Vulcan XC72R (Cabot), and 625 μL of Na?on (5 wt % Aldrich) were mixed with 2 mL of deionized water and 6 mL of isopropyl alcohol. The suspension was sonicated for 1 h. The ink was then sprayed using an Aerograph onto a 6.25 cm2 Sigracet 10 BC gas di?usion layer until a 4.4 mg/cm2 catalyst/C loading was reached. The resulting electrode was hot-pressed with the Na?on 115 membrane with the transferred anode, with a pressure of 160 kg/cm2 for 90 s at 135 °C. The resulting gas di?usion electrode (GDE) was hot pressed onto a pretreated Na?on 115 membrane using the same procedure as for decal transfer. 4.3. Single-Cell Testing. MEAs were assembled in a 6.25 cm2 single cell, having respectively Ti- and gold-coated stainless steel anode and cathode end monopolar plates. Deionized water was pumped through the anode at a ?ow rate of 200 mL/h. The cathode was previously ?ooded with deionized water. The cell was assembled with ?uorinated ethylene propylene gaskets to give a compression of 43 ± 2% and assembled with a torque of 10 N m. MEAs were conditioned at 80 °C for at least 24 h, running with a current density of 1 A/cm2, until the variation of the steady-state potential was less than 1 mV/ min. For measurements at 100 °C the pressure was 2 bar absolute. Polarization curves were recorded on a Bio-Logic SP-150 potentiostat2630DOI: 10.1021/acscatal.5b02443 ACS Catal. 2016, 6, 2626?2631

and the thickness of the FeS2/C catalyst layer is ?30?40 μm. No signi?cant modi?cation of the electrodes or the membrane could be observed; however, the FeS2/C electrode was delaminated from the Na?on membrane. Hence, an optimization of the MEA performance can be expected through the optimization of the ink composition and/or hot-pressing procedure for a better adhesion of the electrode onto the membrane. The durability of materials comprising the membrane electrode assembly is one of the critical concerns in the context of future PEM electrolyzer commercialization. An MEA with a FeS2 cathode was tested for more than 100 h in an electrolyzer, and the performance was found to be steady, which con?rmed the stability already observed in ex situ measurements. The stability of the FeS2 nanoparticles in acidic media (0.1 mol/L H2SO4) was investigated, and no visual degradation or change in the X-ray di?raction pattern was observed.

3. CONCLUSIONS In conclusion, we have demonstrated that the polyol procedure can be used to prepare pure greigite and pyrite nanoparticles at low temperature from cheap and abundant materials. The HER has been evidenced and operated at current density for these nanomaterials, which exhibit remarkable stability over long-

ACS Catalysiswith a 20 A booster by ?rst increasing and then decreasing (shown) the current density from 0 to 2 A/cm2 until the potential stabilized.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02443. Details of electrochemical measurements and additional material characterizations (PDF)